Generally, lubricating oils includes base oils and additives for improving the physical properties of the base oils, and the base oils are typically classified to mineral oils and synthetic oils. As the importance of energy efficiency and environmental pollution is recently emphasized, a demand for sustainable oils, that is being able to be used long-term, has increased. Therefore, industrial field has been increasingly demanded, synthetic oils having excellent properties such as low-temperature fluidity, thermal and oxidative stability, low evaporation losses at high temperatures, high viscosity index, durability on friction, hydrolytic stability, resistance to corrosion, in place of mineral oils. Also, with the regulation of energy efficiency and environmental protection being reinforced, the synthetic lubricating oils are increasingly used in the fields of vehicles and aircrafts.
In vehicle fields, the lubricating oils are used as engine oils, brake oils or lubricating greases, and specifically the engine oils are used as 2-stroke oils, 4-stroke oils or gear oils. In aircrafts fields, the lubricating oils are used as turbine oil, piston engine oils, hydraulic fluids, lubricating greases. While in other industrial fields, the lubricating oils are used as turbine oils, gear oils, bearing and circulation oils, compressor oils, hydraulic oils, metal working fluids, the thermal shearing and insulating oil or lubricating grease. The lubricating oils require various properties depending on the respective purposes. Recently, in accordance with the high performance and high output of an internal combustion engine, are required the lubricating oils having excellent properties such as wear resistance of the internal combustion engine components, heat resistance, sludge resistance, lubricant consumption characteristics or fuel efficiency. Specifically, in order to improve the durability of the internal combustion engine. It is necessary to prevent the lubricating oil reduction and the viscosity decrease of the lubricating oil by shearing of the lubricating oils. Also, in order to improve fuel efficiency, it is necessary to increase the low-temperature viscosity of the lubricating oils. Accordingly, the lubricating oils of long-life, that is, low-evaporation and low-temperature viscosity is required in grease lubricating oils, engine oils, gear oils or hydraulic oils etc.
The synthetic lubricating oil mainly used as an automotive gear oil or engine oil, industrial lubricating oil or hydraulic oil etc, is poly-α-olefin (PAO) (See: U.S. Pat. No. 3,780,128, U.S. Pat. No. 4,032,591, Japanese Unexamined-Publication H1-163136 etc). The POA can be obtained by oligomerization of higher alphaolefin, under an acid catalyst, however, it has a drawback that raw materials of linear alpha-olefin (LAO) is expensive. On the other hands, Japanese Unexamined-Publication 1982-117595 discloses a preparing method for synthetic lubricating oils by copolymerizing the ethylene and alpha-olefin, the synthetic lubricating oils having excellent properties in view of viscosity index, oxidation stability, shear stability, heat resistance etc. In the copolymerization of ethylene and alpha-olefin, is used a catalyst composition composed by titanium compound and organic aluminum compound, generally known as the 1st generation catalyst. The titanium compound catalyst has large catalystic activity, but molecular weight of the copolymer obtained has wide range and low Irregularities. Accordingly, it is difficult to obtain high flash point products which are useful for lubricating oils, lubricating oil additives, fuel oil additives etc, and in case of high viscosity products, cost thereof is high so that it is not practical. Also, U.S. Pat. No. 5,767,331 discloses a method for copolymerizing the ethylene and alpha-olefin, specifically copolymerizing ethylene and propylene, by using vanadium-based catalyst composition containing vanadium compound and organic aluminum compound. The copolymer prepared by using vanadium-based catalyst composition has narrow molecular weight distribution and superior uniformity. But the copolymer prepared by using vanadium-based catalyst composition generally has very low polymerization activity, and accompanies large amount of catalyst sludge so that it has a drawback of requiring the additional de-catalytic process, which is common problem on the 1st generation catalyst such as Ziegler-Natta catalyst. In addition, Japanese Unexamined-Publication S61-221207, Japanese Unexamined-Publication H7-121969 etc. disclose a method for preparing the copolymer with high polymerization activity by using a catalyst system composed by metallocene compound such as zirconocene and so on and organoaluminum oxy-compound, and Japanese Patent 2796376 discloses a method for preparing synthetic lubricating oils by copolymerizing the ethylene and alpha-olefin, by using a catalyst system composed by specific metallocene catalyst and organoaluminum oxy-compound.
In order to overcome the disadvantages of the 1st generation catalyst such as a Ziegler-Natta catalyst, the use of bis-indenyl metallocene catalyst has been attempted. The bis-indenyl metallocene catalyst forms composition of more homogeneous (pseudo random) chain structure. However, it is difficult to prepare pure racemic bis-indenyl catalyst, so that not only its preparing cost is high but also, if the pure catalyst is obtained, it cannot be avoided for part of the catalyst to be switched to the meso-derivatives. Also, in case of the catalyst, since the steric hindrance increases in metal center after the 2,1-insertion, there is a problem of decreasing the molecular weight of the produced polymer. The catalyst increases only ethylene reactivity, and the random copolymer produced by using the catalyst contains an unsaturated double bond, to lower the thermal stability and durability of the synthetic lubricating oils. Accordingly, when the catalyst is used there is a drawback that a process of hydrogen addition using hydrogen should be additionally carried out for removing the unsaturated double bonds. Also, Literature of Angew. Chem. Int. Ed. 1998, Vol. 37, No. 7, p 922-925 (Leclerc and Waymouth) discloses a method for copolymerizing ethylene and propylene using metallocene compound having cyclopentadiene (Cp) ligand and fluorene (Flu) ligand. Particularly, the Literature discloses zirconium metallocene catalyst containing unsubstituted Cp, 3-methyl Cp, 3-tert-butyl Cp and 3,4-dimethyl Cp ligands, however, the catalyst is only partially useful for the preparation of quasi-random polymers, and there is significant degree of randomization in the polymers produced by the catalyst.
Like the above, the metallocene catalysts are known to be useful in a variety of polymerization processes, however, up to now, a preparation method of olefin copolymer having superior random degree of ethylene and propylene being equally alternately reacting, has not been known. Therefore, the improved process for the preparation of a random olefin copolymer has been desired. Further, recently considering environmental issues, fuel consumption, energy saving, etc., the demand is increased for synthetic lubricating oils of PAO or ethylene/propylene copolymer having superior durability, low-temperature viscosity (viscosity index) characteristic, heat and oxidation stability.